First of all, 15% casualties is not a low rate. Out of you and 5 of your friends, that would be about one of you dead or wounded. I doubt that you would consider that trivial. That said, I have also read that in many battles of antiquity, casualty rates were quite low --
until one side broke and ran. Then the slaughter began, and it could be gruesome.
Think about it -- killing people in hand to hand combat is a piecemeal, retail affair when they are similarly armed, and fighting back. Only the ones who are in contact with the enemy could do the killing, and it takes time, you get tired, and may well be the one getting killed or wounded yourself if you are not careful! So it's a slow business, and this stuff about heroes killing 5 men at a single blow is just that, stuff.
However, at some point, and never having been in combat myself I have not experienced it, one side, or actually a small fraction of one side, decides they are going to lose. And they turn around. [this, by the way, is why the Optio is stationed -behind- the Century in battle formation, to stop routs from happening. Routs start from the rear] And fear is tremendously contagious. The battle line "unzip"s as men's flanks are no longer protected, order crumbles, and then suddenly the killing changes from retail to wholesale as you no longer have to beat a man's defences to kill him, you can just stab him in the back. And this is the moment the Cavalry lives for: to be able to run down and slaughter fleeing enemies. By the way, this is shown very nicely in the computer game Rome: Total War.
Anyway, back to the original question about Romans vs Romans and low casulties: I don't know. But since this is a board, I can speculate. Romans were well armored, so they were probably less vulnerable to missile fire, and slingers and archers were usually auxiliaries anyway, so may (?? experten?) not have played too much of a role in Roman Civil wars. This may have included less vulnerability to Pila as well. A soldier's scutum may have been knocked out of action, but with discipline he may have been able to rotate to the rear ranks for repair or replacement.
Also, the Roman army was generally known for -not- breaking and running, so it is possible that Roman Armies just ground on each other all day with low casualties. Sometimes. However, I have read of fearsome casualties in the Roman Civil wars. See, for example,
The Battle of Pharsalus where Pompey's side lost an estimated 18,000 out of 60,000. Anyway I have rambled on quote long enough, perhaps someone who knows what they are talking about should take over.
rkmvca/rich klein